Center of Mass (COM)
What is Center of Mass (COM) in Humanoid Robotics?
The point where the total mass of a robot can be considered to be concentrated.
Understanding and controlling the center of mass is crucial for maintaining balance in humanoid robots, especially during dynamic movements like walking or reaching.
How Center of Mass (COM) Works
The center of mass is calculated by considering the mass and position of every component in the robot. For a humanoid robot, the COM shifts constantly as arms move, legs swing, and the torso rotates. The control system continuously calculates the COM position in real-time using joint angle data and the known mass distribution of each body segment. To maintain balance during walking, the robot must keep its COM projection (the point directly below it) within the support polygon formed by the feet on the ground. Sensors provide feedback, and the control algorithm adjusts joint positions to shift the COM as needed - leaning forward when accelerating, shifting weight over the supporting leg during a step.
Types of Center of Mass (COM)
- Static COM: Center of mass when robot is stationary, calculated from fixed geometry
- Dynamic COM: Constantly changing COM during movement, requires real-time computation
- Projected COM: The vertical projection of COM onto the ground plane, critical for balance
- Whole-Body COM: Total body center of mass including all segments
- Segmental COM: Center of mass of individual body parts like arms or legs, used in calculating whole-body COM
Applications in Humanoid Robots
COM control is fundamental to all humanoid robot balance and locomotion. During walking, the robot shifts its COM over the stance leg before lifting the other foot. When picking up objects, the robot adjusts posture to compensate for added weight and keep COM stable. In dynamic movements like jumping, precise COM control determines landing stability. Manipulation tasks while standing require continuous COM adjustment to counterbalance arm movements. Recovery from pushes or disturbances involves rapid COM repositioning to prevent falls.
